Methods for forming backside alignment markers useable in semiconductor lithography

ABSTRACT

Disclosed herein are methods for forming photolithography alignment markers on the back side of a substrate, such as a crystalline silicon substrate used in the manufacture of semiconductor integrated circuits. According to the disclosed techniques, laser radiation is used to remove the material (e.g., silicon) from the back side of a substrate to form the back side alignment markers at specified areas. Such removal can comprise the use of laser ablation or laser-assisted etching. The substrate is placed on a motor-controlled substrate holding mechanism in a laser removal chamber, and the areas are automatically moved underneath the laser radiation to removal the material. The substrate holding mechanism can comprise a standard chuck (in which case use of a protective layer on the front side of the substrate is preferred), or a substrate clamping assembly which suspends the substrate at its edges (in which case the protective layer is not necessary). Alternatively, a stencil having holes corresponding to the shape of the back side alignment markers can be placed over the back side of the substrate to mitigate the need to move the substrate to the areas with precision.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/840,733, filed May 6, 2004, which is incorporated herein by referencein its entirety and to which priority is claimed.

This application is related to U.S. patent application Ser. No.10/840,324, filed May 6, 2004, and which is incorporated by reference inits entirety.

FIELD OF THE INVENTION

Embodiments of this invention relate to improved methods for formingback side alignment markers useable in semiconductor photolithography.

BACKGROUND

When fabricating an integrated circuit, and as is well known, a seriesof layers are deposited on a substrate (usually a crystalline siliconsubstrate) and are patterned and etched to form a circuit. For thecircuit to work properly, it is important that each subsequent layer bealigned with the previously formed layer or layers, at least within somepermissible tolerance.

To align the various layers, and referring to FIG. 1A, a substrate 12having a photoresist applied thereon (not shown) is placed in aphotolithographic chamber 10, sometimes referred to as a “stepper” or“scanner.” In the stepper 10, a mask or reticle 27 is used to patternthe photoresist. As the patterned photoresist ultimately dictates thepositioning of the underlying circuit layer to be etched, its alignmentis critical.

To bring the substrate 12 into alignment with the mask 27, an image ofsome structure on the mask and some structure 24 on the wafer arecompared using well-known optical analysis equipment 14, with suchimages being received by optical sensors 20. If alignment is needed, theoptical analysis equipment 14 can control the positioning of a chuck 16on which the substrate 12 sits via motor stages 18, which, for example,can move the chuck 16 along the X-axis, Y-axis, or rotational θ-axis asappropriate. Such alignment is usually assessed at numerous locationsaround the substrate 12's perimeter, which accordingly requiresreference to a plurality of alignment structures 24 on the substrate 12,as shown in FIG. 1B. However, reference to a single alignment markercould also be used.

Although alignment structures 24 can constitute an actual active portionof the circuit being fabricated, a dedicated inactive structure isusually formed for this purpose—what is referred to as an alignmentmarker. Referring to FIG. 1B, such alignment markers 24 are typicallyformed outside of the active integrated circuit area 22 on the wafer,i.e., in the area in which the substrate will be scribed or “diced” forlater insertion into packages. A simple “cross” pattern is illustratedfor the alignment marker 24, but as one skilled in the art willunderstand, such markers can come in a variety of different shapes andsizes (e.g., chevrons, gratings, squares, etc.), depending on thealignment task be performed. Typically, more than one alignment marker24 is fabricated on the substrate 12 as shown, which may range fromapproximately 20 to 500 microns in size.

However, alignment markers appearing on the front side of the substratesuffer from the problem that such markers may eventually become coveredwith opaque materials during later processing steps, and hence maybecome difficult for the optical sensors 20 to “see,” as discussed atlength in above-incorporated U.S. patent application Ser. No.10/840,324.

Accordingly, the prior art has experimented with the use of back sidealignment markers. As their name suggests, back side alignment markersare located on the opposite side of the substrate from the front sidewhere the active circuitry is formed. The processing steps used to formthe active circuitry on the front side generally do not appreciablyaffect the back side; for example, materials deposited on the front sideof the substrate will generally not find their way to the back side,except in trace amounts. Accordingly, back side alignment markersgenerally remain unaffected during processing of the substrate, andtherefore remain visible to the optical sensors 20 for alignmentpurposes.

An exemplary stepper chamber 30 relying on the use of back sidealignment markers 27 is shown in FIG. 2. Such a chamber is generallysimilar to the chamber 10 of FIG. 1, but includes holes 17 in the bottomof the chuck 16 aligned with the backside alignment markers 27. Mirrors15 direct light between the back side alignment markers 27 and theoptical sensors 20 to allow the alignment markers 27 to be “seen” foralignment purposes. Alternatively, channels through the chuck andparallel to the substrate 12's surface can carry the optical pathbetween the back side alignment markers 27 and the optical sensors 20,such as is disclosed in http://www.minanet.com/documents/ASML.pdf (Sep.25, 2003), which is submitted herewith and which is incorporated byreference in its entirety.

However, back side alignment markers still suffer from processingdifficulties, as illustrated by the process of FIG. 3, which shows howsuch back side alignment markers are traditionally formed. FIG. 3A showsin cross section a blank or stating substrate 12, which again is usuallya silicon crystalline substrate. The substrate 12 has a front side 12 aand a bottom side 12 b. Prior to fabrication of the integrated circuiton the front side 12 a, the front side is highly polished, rendering thefront side 12 a to near perfect smoothness at the atomic level that isappropriate for the formation of transistors and the like. The back side12 b is generally also smooth, but usually not as smooth as the frontside 12 a.

Traditionally, the back side alignment markers 27 are formed usingtraditional photolithography techniques. However, care must be taken toprotect the near-perfectly smooth front side 12 a, as this surface iseasily scratched. If scratched, the electrical structures (such astransistors) eventually formed at the front side 12 a will “leak”current and otherwise may perform poorly from an electrical standpoint.Accordingly, before formation of the back side alignment markers 27, aprotective layer 40 is formed on the front side 12 a, as shown in FIG.3B. Typically, this protective layer 40 constitutes a silicon dioxide orsilicon nitride layer.

With the front side 12 a protected, photolithography processing on theback side 12 b can now begin. Accordingly, and referring to FIG. 3C, thesubstrate 12 is inverted and placed front side 12 a down onto a worksurface 42, which varies throughout the back side alignment markerformation process but which initially would comprise a photoresistspinning apparatus. A photoresist 41 is deposited (spun) on the backside 12 b, and is moved to a stepper apparatus, where it is exposedusing a mask having the desired back side alignment marker 27 pattern(not shown) and developed to expose the back side 12 b through thephotoresist (FIG. 3D). The substrate 12 is then moved to an etchingchamber (not shown) where the alignment marker is etched into the backside 12 b using the remaining photoresist as a masking layer (FIG. 3E).Thereafter, the remaining photoresist 41 is stripped (not shown), andthe protective layer 40 is etched away (not shown), thus leaving theback side alignment marker 27 on the back side 12 b of the substrate 12(FIGS. 3F and 3G). Thereafter, the substrate 12 can be processed to forman integrated circuit, using the back side alignment marker 27 (ormarkers) to align the substrate 12 with each mask during sensitivephotolithography steps in chamber 30 (FIG. 2).

However, it should be appreciated from the foregoing that formation ofthe back side alignment markers 27 involves a lot ofpreparation—protective layer formation, photoresist deposition,patterning and removal, etching, removal of these layers, etc.—beforeprocessing of the substrate 12 can begin in earnest to form activeuseful structures on the front side 12 a. Accordingly, the art would bebenefited by improved methods for forming back side alignment markers,and in particular methods that forego these additional steps. Thisdisclosure provides solutions.

SUMMARY

Disclosed herein are methods for forming photolithography alignmentmarkers on the back side of a substrate, such as a crystalline siliconsubstrate used in the manufacture of semiconductor integrated circuits.According to the disclosed techniques, laser radiation is used to removethe material (e.g., silicon) from the back side of a substrate to formthe back side alignment markers at specified areas. Such removal cancomprise the use of laser ablation or laser-assisted etching. Thesubstrate is placed on a motor-controlled substrate holding mechanism ina laser removal chamber, and the areas are automatically movedunderneath the laser radiation to removal the material. The substrateholding mechanism can comprise a standard chuck (in which case use of aprotective layer on the front side of the substrate is preferred), or asubstrate clamping assembly which suspends the substrate at its edges(in which case the protective layer is not necessary). Alternatively, astencil having holes corresponding to the shape of the back sidealignment markers can be placed over the back side of the substrate tomitigate the need to move the substrate to the areas with precision.Using the disclosed techniques, a separate photolithography step to formthe back side alignment markers is not necessary, and additionally theneed to use a protective layer on the front side of the substrate ispotentially unnecessary, saving time and cost, and reducing potentialsources of contamination.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the inventive aspects of this disclosure will be bestunderstood with reference to the following detailed description, whenread in conjunction with the accompanying drawings, in which:

FIG. 1A illustrates a prior art photolithographic stepper for opticallysensing front side alignment markers on a substrate, and FIG. 1Billustrates a top-down view of the alignment markers on the substrate.

FIG. 2 illustrates a prior art photolithographic stepper for opticallysensing back side alignment markers on a substrate.

FIGS. 3A-3G illustrate a prior art process for forming back sidealignment markers using photolithography and a protective layer for thefront side of the substrate.

FIG. 4 illustrates a prior art laser ablation or laser-assisted etchchamber useful in accordance with the disclosed technique for formingback side alignment markers without the need for a photolithographystep.

FIGS. 5A-5D illustrate using cross-sectional views the disclosedtechnique for forming back side alignment markers without the need for aphotolithography step.

FIG. 6 illustrates a modified laser ablation or laser-assisted etchchamber having a clamping assembly for suspending the substrate, thusmitigating the need to provide a protective layer on the front side ofthe substrate.

FIG. 7 illustrates a top down view of the clamping assembly of FIG. 6.

FIG. 8 illustrates a modification in which a stencil is use to mask theback side of the substrate and useful for forming the back sidealignment markers with improved precision.

FIG. 9 illustrates a back side alignment stepper which has been modifiedto include lasers for writing the back side alignment markers.

DETAILED DESCRIPTION

In one embodiment of the disclosed invention, laser-assisted etching orlaser ablation is used to form back side alignment markers. Thedisclosed technique is beneficial over the prior art in that it does notrequire the use of photolithography to form the back side alignmentmarkers, and additionally in some embodiments does not require theprovision of a protective layer on the front side of the substrate. Itshould be noted that both laser-assisted etching and laser ablation arewell-known techniques that have been used to etch materials onintegrated circuits. Accordingly, only basic aspects of these techniquesare discussed, with the focus of the discussion centering on aspectsrelevant to the back side alignment marker issues discussed earlier.

In FIG. 4, a laser-assisted etch/laser ablation chamber 50 is shown, andwhich is used in this embodiment to form the back side alignmentmarkers. The chamber 50 includes an optical sensor 51, a laser 52, alens or lenses 54, a computer 56, motor stages 58, a gas inlet port 60coupled to an etchant gas source 62 via a valve 64, and a purge pump 73.Chambers with these components are well known, and will vary in designdepending on whether laser-assisted etching or laser ablation techniquesare used. Because a gas inlet port 60, an etchant gas source 62, and apurge pump 73 are shown, chamber 50 as illustrated more accuratelyrepresents a laser assisted-etching chamber. Were laser ablation to beused for the application in question, such gas- and etching-specificstructures may not be necessary. Further details concerninglaser-assisted etch/laser ablation chambers 50 are described inabove-incorporated U.S. patent application Ser. No. 10/840,324. Alsopresent is the substrate 70 to be processed, which sits upon a substrateholding mechanism 71, such as a chuck 72 or a clamp assembly 79 to bedescribed in further detail later.

The laser 52 is used to etch or ablate the bulk substrate 70 material onthe backside 70 b without the need to practice the photolithographysteps of the prior art (photoresist deposition, exposure, cleaning andremoval of the photoresist, etc.). In this regard, the substrate 70 isinitially aligned front side 70 a down in the chamber 50. This alignmentcan be relatively crude (e.g., +/−20 microns), and need not be assophisticated as the alignment schemes used to align the circuit layersin the device. Thus, initial alignment need only be +/−5 microns forexample, and can be performed manually, via operator visual inspectionthrough a microscope, or by automated optical detection schemes, such asautomated detection of the edges of the substrate 70 via the use of theoptical sensor 51.

Once aligned, computer 56 executes a program specific for the substrate70 in question, and armed with knowledge of the X, Y coordinates ofwhere the back side alignment markers 27 are to be fabricated on thesubstrate 70. Accordingly, the computer 56, moving the substrate holdingmechanism 72 via motor stages 58, brings the desired back side alignmentmarker areas 75 into alignment (FIGS. 5A and 5B) and engages the laser52 to form laser radiation 53 to etch or ablate the back side 70 b ofthe substrate 70, e.g., of crystalline silicon (FIGS. 5C and 5D).Thereafter, the substrate 70 can be cleaned if necessary, and as itwould be cleaned in any event prior to further processing of activestructures on the front side 70 a of the substrate 70. In short, theback side alignment markers 27 are formed without photolithography andall of the steps entailed therein.

As noted earlier, techniques for using laser-assisted etching and laserablatement of materials on semiconductor substrates are well known, andhence are not reiterated in much detail herein. Considerations relevantto such selective area processing can be found in Thin Film ProcessesII, (ed. John L. Vossen & Werner Kern), pp. 621-670, 749-856 (AcademicPress 1991), which is submitted herewith and which is incorporatedherein by reference.

Laser ablation is preferably accomplished using an excimer, YAG, orND-YAG laser which essentially vaporizes the metal layer 30 or othermaterial where it is focused. Suitable ND-YAG lasers have wavelengths of355 nm, and suitable excimer lasers have wavelengths of 193 nm or 248nm. Power levels for such lasers are typically in the 1-Watt range.Laser ablation is simpler to implement, and will remove materialrelatively quickly, but is more difficult to control. Moreover, thevaporized material may need to be cleaned from the substrate 70'ssurface. This being said however, laser ablatement can be a suitablechoice for forming back side alignment markers in the substrate 70 insome applications. Exemplary excimer lasers include the PL-1500A ExcimerLaser manufactured by Potomac Photonics, Inc., and the xsie200 ExcimerLaser manufactured by Xsil Ltd.. An exemplary YAG laser suitable forablation comprises that LAM 66 manufactured by Heidelberg InstrumentsMikrotechnik GmbH. Further details regarding considerations for laserablation can be found athttp://www.me.mtu.edu/˜microweb/chap4/ch4-2.htm,http://www.me.mtu.edu/˜microweb/graph/laser/fluencejpg, andhttp://www.me.mtu.edu/˜microweb/graph/laser/specmetjpg, which aresubmitted herewith and which are incorporated by reference in theirentireties.

Laser-assisted etching, by contrast, is slower, but better controlled,and hence is preferred for the application in question. Inlaser-assisted etching, an etchant gas is introduced into the chamber 50from an etchant gas source 62 through valve 64 and gas inlet port 60.The etchant gas is preferably introduced into the chamber 50 as shownproximate to and parallel with the substrate 70's surface. Interactionof the laser light and the etchant gas produces a controlled reaction atthe surface of the substrate 70 to remove the material in question. Ofcourse, the etchant gas to be used for a particular application, as wellas the laser 52 parameters (wavelength; power; spot size) will depend onthe composition of the substrate 70, but again such laser-assistedprocesses are well known. If silicon or polysilicon is being etched, SF₆would be a suitable etchant gas and would be used in conjunction with alaser having approximately a 10um wavelength. Other etchant gases andassociated laser wavelengths suitable for etching silicon can be foundin the above-referenced Thin Film Processes book incorporated above atpage 832. An exemplary laser-assist etch chamber 50 can comprise thelaser etch and deposition chamber published at http://www.mesofab.com,which is submitted herewith and which is incorporated herein byreference.

Because the area 75 in which material will be removed will generally berelatively large compared to the spot size of the laser 52, removal willpreferably be accomplished by rastering the area 75 underneath the laser52. The laser 52 can either run continuously, or can be turned on andoff at each rastered location. Alternatively, if the laser spot size islarge enough and comparable with the size of area 75, rastering may notbe necessary.

As noted earlier, it is important during formation of the back sidealignment markers 27 that the front side 70 a of the substrate 70 not bedamaged. In the prior art, protection of the front side 70 a surface wasprovided by a protective layer (40; FIG. 3B). Using the laser-assistedetch/laser ablation chamber 50 shown in FIG. 4, provision of such aprotective layer would be preferred, as the front side 70 a of the wafercomes into contact with the chuck 72.

However, in a preferred embodiment, provision of a front side 70 aprotective surface is rendered unnecessary by making modifications tothe laser-assisted etch/laser ablation chamber 50. Specifically, and asshown in FIGS. 6 and 7, the chamber 50 a has been modified by exchanginga clamp assembly 79 for the chuck 72 of FIG. 4. The clamp assembly 79has the same translational capabilities as the chuck 72, and again iscontrolled in its movement by the computer 56 via motor stages 58.However, through the use of the clamp assembly 79, the substrate 70 issuspended within the modified chamber 50 a such that its front side 70 adoes not come into contact with the clamp assembly and does notsubstantially come into contact with any work surface, except at certainnon-critical points 85 along its edge not otherwise suitable for theformation of active circuitry. Accordingly, the back side alignmentmarkers 27 can be fabricated in modified chamber 50 a without the needto provide a protective layer 40 on the front side 70 a. This save aprocess step, and allows for processing of active structures on thesubstrate 70 essentially immediately after the back side alignmentmarkers 27 are formed.

As best shown in FIG. 7, the clamp assembly 79 spans underneath thesubstrate 70 at cross member 83 and rises along toward the sides of thesubstrate at risers 82. Clamp arms 84 are coupled to the risers 82 andcontain a bottom arms and top arms, which contact the front side 70 aand back side 70 b of the substrate 70 respectively. The clamp arms 84are coupled to the risers 82 via suitable mechanisms 81. Thesemechanisms 81 could comprise the spring mechanism for biasing the topand bottom clamp arms 84 together to pin the substrate 70 therebetween,and additionally can incorporate motors to allow the substrate 70 to berotated around an axis (φ) joining the two mechanisms 81. This allowsthe wafers to be loaded front side 70 a up, and then rotated to bringthe back side 70 b up, thus providing operational flexibility.Rotational capability also allows for front side laser-based processingas well, e.g., to form alignment markers on the front side 70 a as wellas the back side 70 b. Such rotation and activation of the motors can beaccomplished using the computer 56 and the motor stage controls 58, withwiring to the motors 81 being routed though the body of the cross member83 and the risers 82.

In other embodiments, the substrate 70 need not be flipped as in chamber50 a, but instead can sit flat on its back side 70 b when being writtento. In such an embodiment, and as shown in FIG. 9, a back side alignmentchamber 30 a or stepper such as those discussed above (e.g., FIG. 2),can be retrofitted with a laser or lasers 52. The laser 52 a will allowthe back side alignment marker to be written (i.e., by laser ablation orlaser-assisted etching) using the same optical path used to “see” thealignment markers once they are formed. The lasers 52 may either appearproximate to the front side 70 a of the substrate (52 a) or proximate tothe back side 70 b of the substrate 70 b (52 b). Through thismodification, the same tool can be used both for writing the back sidealignment markers and as a photolithographic stepper that uses thosemarkers for alignment purposes. Beneficially, through this modification,a protective layer 40 over the front side 70 a of the substrate need notbe used when writing the back side alignment markers with the laser.

In an alternative embodiment, the need for the computer 56 to know theprecise X, Y coordinates of the areas 75 to be removed on the back side70 b is mitigated by the use of a stencil 90, as shown in FIG. 8.According to this alternative, the stencil 90 is aligned with thesubstrate 70, and contains holes 92 which correspond to the desiredshape of the backside alignment markers 27. The stencil 90 is raised adistance ‘d’ away from the surface of the substrate 70 by spacers 91,which distance might range from approximately 2 to 10 microns. Usingthis approach, the laser 52 can be rastered over the entire surface ofthe stencil 90, yet will only have effect to remove the material exposedon the back side 70 b where it is exposed through the holes 92.Accordingly, once the stencil 90 is appropriately aligned with thesubstrate 70, using any of the techniques mentioned earlier, radiation53 from the laser 52 can be appropriately directed to form the back sidealignment markers 27 into the desired shape.

Thus, the stencil 90 ensures good alignment of the radiation 53 with thedesired area 75, making laser alignment and spot size considerationsless critical. (Indeed, the use of a laser in conjunction with a stenciloverlying the wafer has utility to clearing materials over and aboveclearing the alignment markers, and can be used for patterning activecircuits as well). Because the back side alignment markers arerelatively large, diffractive effects occurring at the edges of theholes 92 of the stencil 90 should not cause a problem, although opticalproximity corrective measures could be incorporated into the stencil 90if necessary. If used in a laser-assisted etch application, a materialshould be chosen for the stencil that will not react to the etchantgases in question. For example, for a silicon etchant, quartz (silicondioxide) would be a good choice for material for the stencil 90.Likewise, in a laser ablation application, a material should be chosenwhich will remain impervious to the laser radiation in question.

Although the stencil 90 is disclosed in the Figures in conjunction withchuck 72, it should be understood that the stencil 90 can also be usedwith the clamp assembly 79 disclosed earlier (FIGS. 6 and 7). If soused, the stencil 90 and spacers 91 would also be clamped between thetop and bottom clamp arms 84 of the clamp assembly 79.

Although the disclosed laser-assisted/laser ablation techniques havebeen disclosed as useful in the context of forming back side alignmentmarkers, it should be understood that the disclosed techniques can beused to form front side alignment markers, and/or to remove circuitlayers from the front side.

Moreover, while particularly useful to the clearing of materials onsemiconductor integrated circuit substrates, the disclosed techniquescan have application to other types of substrates and other types ofprocesses.

While it is preferred to use radiation, and specifically laserradiation, to etch the substrate to form the back side alignmentmarkers, this is not strictly necessary. One skilled in the art willrealize that other techniques for selectively removing discrete areas ofmaterials without the use of a photoresist exist in the art, and thesecould be used as well. For example, an electron or other particle beam(e.g., an ion beam) could be used in much the same way as the disclosedlaser radiation 53 is used to remove the substrate material by the useof a rastered beam to directly remove the material without the need forphotoresist or photolithography. The use of such alternative beams canalso be accompanied by the use of a stencil as disclosed herein. Again,processes for using electron or particle beams to remove materials fromsemiconductor integrated circuits are well known, and can be found inthe Thin Film Processes book incorporated above.

“Circuit layer” as used herein can comprise any layer used in theformation of integrated circuits on the front side of the substrate,including conductive layers, semiconductive layers, or insulatinglayers, doped regions of the silicon, etc.

It should be understood that the inventive concepts disclosed herein arecapable of many modifications. To the extent such modifications fallwithin the scope of the appended claims and their equivalents, they areintended to be covered by this patent.

1. A laser removal tool, comprising: a laser for producing radiationimpingent on a substrate, wherein the impingent radiation removesmaterial from a substrate; and a moveable assembly for suspending thesubstrate in the tool such that the substrate does not substantiallycome into contact with a work surface.
 2. The tool of claim 1, whereinthe impingent radiation removes the material by ablation.
 3. The tool ofclaim 1, wherein tool further comprises an input port for an etchantgas, and wherein the impingent radiation interacts with the etchant gasto remove the material.
 4. The tool of claim 1, further comprisingmotors to rotate the suspended substrate around a horizontal axis. 5.The tool of claim 1, wherein the moveable assembly comprises clamps, andwherein the substrate is suspended in the tool using the clamps.
 6. Alaser removal tool, comprising: a moveable chuck for holding asubstrate, wherein a back side of the substrate contacts the chuck; andat least one laser for producing radiation impingent on the back side ofthe substrate, wherein the impingent radiation removes material from theback side of the substrate.
 7. The tool of claim 1, wherein theimpingent radiation removes the material by ablation.
 8. The tool ofclaim 1, wherein tool further comprises an input port for an etchantgas, and wherein the impingent radiation interacts with the etchant gasto remove the material.
 9. The tool of claim 1, wherein the chuckcomprises at least one optical channels through which the radiationimpinges on the back side of the substrate.
 10. The tool of claim 1,wherein the laser is proximate to the front side of the substrate. 11.The tool of claim 1, wherein the laser is proximate to a back side ofthe substrate.